Background
For all river basins globally, the reservoir capacity is declining by around 5% compared to the installed capacity (Wisser et al., 2013). A study, for 285 dams, was made by the Italian national Committee for Large Dams (ITCOLD) in 2009. It indicates that the loss of storage in 53% of investigated hydroelectric reservoirs in Italy has an average of 47% due to sedimentation. The other dams have a loss in storage less than 5% (Bizzini et al., 2009).
Reservoir sedimentation creates environmental challenges. For instance, reduction of river bed slope upstream of the reservoir (low velocity), influencing water quality due to contaminated sediments stored in the reservoir, reducing resting habitat for fish due to fine sediment deposition, bed degradation downstream of the dam and coarsening the riverbed which may not be suitable for spawning (Spreafico, 2007). Moreover, the reservoir sedimentation upturns the probability of flood inundation area at the upstream reach of reservoir tail due to bed aggradation (Kantoush et al., 2010).
Detaining of sediment due to dammed rivers may generate unfavourable morphological changes downstream of the dam (e.g. bed degradation, bank erosion and scour formation) and contributes also to coastal erosion (Guertault et al., 2014). Around 53% of global sediment fluxes in regulated basins are potentially trapped in reservoirs. This may influence the downstream morphological behaviour and coastal area that rely on riverine sediment supply (Kondolf et al., 2014).
Therefore, sedimentation creates a long-term economic loss and needs to be mitigated (Liu et al., 2004). It is vital to include or improve the sediment management and update reservoir operations rules. To realize which factors can improve passing the sediment to the downstream reach of the dam. This knowledge will help to increase the probable life span of a reservoir. Moreover, it will lead to measures that can be taken against reservoir sedimentation, water shortage and erosion of coast and river banks. These measures are called sediment management practices.
1.1 Sediment management
In the recent reservoir design and implementation, the cumulative reservoir sedimentation, during the assessed economic period of the reservoir, is estimated and added to the reservoir storage within the dead storage.There are some additional measures which could be considered to mitigate the sedimentation such as sluicing, dredging, flushing, etc (Fruchard and Camenen, 2012).
Flushing is almost dredging for free. The cost inherent to flushing of 1000 ton of sediment is negligible, compared to the dredging cost for the same amount. There are two types of flushing:
- Hard Flushing: It is a drawdown of reservoir water level to a minimum. It is very effective in terms of flushing sediment, but it may generate high flow velocities inducing bed or bank erosion and uncontrolled sediment concentration which may directly harm the fish habitats downstream of the dam.
- Environmentally friendly flushing: It is achieved by a limited drawdown of the reservoir water level. The level is determined based on sediment concentration allowed in water to mitigate or avoid the impacts to the downstream ecosystem (Baran and Nasielski, 2011).
Danelli and Peviani (2012) recommended the use of real-time reservoir operation to get a better understanding of sediment transport through the dam. This will be the approach in this study. To investigate the sediment flushing using two different gate operation patterns, Funagira Dam in Japan will serve as a case study.
References
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3. Bizzini, F., et al.. " The silting problem for reservoirs of Italian large dams, 2009.
4. Danelli, A. and Peviani, M., " D6.9 Application of A Morphological Model to Evaluate Downstream Effect of Reservoir Flushing Operation": Transnational Cooperation Program - South East Europe and the European Union,2012.
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13. Sloff, C. J. a. J., H.R.A., " Tenryuu River morphological modelling study. Delft Hydraulics, Report Q2794.00.": Deltares,2001.
14. Spreafico, M., " Environmental impact caused by reservoir sedimentation management - Experiences in the River Rhine Basin", Workshop on Reservoir Sedimentation Management Beijing, China,2007.
15. Wisser, D., et al.. " Beyond peak reservoir storage? A global estimate of declining water storage capacity in large reservoirs", Water Resources Research, 49, 5732-5739, 2013.
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